(a) Technical Field of the Invention
The present invention generally relates to air pumps, and more particularly to a piston assembly for quickly releasing the compressed air and inner pressure inside the cylinder of an air pump.
(b) Description of the Prior Art
When an air pump finishes pumping and is turned off, compressed air and thereby a high pressure are still preserved inside the cylinder of the air pump. When the air pump is later started again and the piston is engaged, the piston is resisted by the preserved high pressure and the air pump would suffer an instantaneous load proportional to the cylinder's inner pressure. If the load is large enough, the air pump might not be able to be started, or the motor of the air pump would burn down.
As shown in FIG. 1, a conventional air pump is therefore provided with a pressure releasing device (A1) that is located either on the cylinder (A2) or along the air pipe (A3). Every time before the air pump is started, a button (A4) of the pressure releasing device (A3) is engaged manually to release the compressed air and the inner pressure inside the cylinder (A2), which is quite troublesome.
Some conventional air pumps are configured with a pressure-sensitive releasing device which would be automatically engaged when the inner pressure inside the cylinder has reached a pre-set threshold. In other words, the inner pressure of the cylinder is always maintained within a safety range. This implies that the inner pressure of cylinder is not entirely removed. The remaining inner pressure inside the cylinder would still cause unstable operation of the piston.
Small air pumps such as those for use in households or with motor vehicles usually do not have a pressure releasing device. Instead, a small through exhaust hole is provided on the cylinder so that a portion of the compressed air is always released via the exhaust hole while the remaining compressed air is pumped out through the air pipe. In other words, this kind of small air pumps sacrifices a portion of the compressed air for the resolution of the start-up inner pressure problem. However, this simple solution is not adequate to high-power and high-pressure air pumps. In addition, as some of the compressed air is wasted, the motor of the air pump has to work longer and consumes more electricity. Also, if the air pump is restarted when the compressed air inside the cylinder has not totally escaped through the exhaust hole, the motor would still suffer an instantaneous load.